1. Technical Field
The present invention relates to controls for electric or fluid actuators, and to conveyances propelled by electric or fluid actuators. More particularly, the present invention relates to dynamically braking conveyances, and to controlling the turns of conveyances that are steered by separately controlling the speeds of the wheels.
2. Description of the Related Art
Conveyances of various types, for transporting people, for material handling, and for propelling self-propelled machinery, have requirements for extremely high maneuverability.
One way to obtain extremely high maneuverability is to separately and variably control the speed and direction of rotation of left and right wheels or other propulsion elements. When the wheels are moving at the same speed, but in opposite directions, the conveyance pivots in a fixed location, giving the ultimate in maneuverability.
A propulsion system using electric or hydraulic motors can provide flexibility of control, but precision of control has been lacking in prior art designs.
Further, achieving high maneuverability by separately controlling the velocity and direction of rotation of the wheels, or other traction elements, may make a conveyance difficult to control, or even dangerous.
For instance, it may be desirable to have the ability to make pivot turns with some conveyances, but it might be dangerous to attempt to make a pivotal turn at full speed.
But, if the rate of change of speed of the individual wheels is limited, then the machine may be sluggish in acceleration, and may be dangerously slow in deceleration.
The problem of controllability is particularly acute in wheelchairs, and the discussion that follows centers on electricallypropelled wheelchairs.
Typically, separate D.C. electric motors have been connected to left and right wheels of a wheelchair by chains or belts, and by friction rollers that have separately engaged the rubber tires of the wheels.
D.C. motors provide both directions of rotation by changing polarity of the driving voltage, and produce rotational speeds that are dependent upon both the driving voltage and the torque required of them.
Manually actuated controls have been used that separately and variably supply electric power from a battery to left and right motors to make changes in speed, and to make turns, including pivot turns.
One popular type of manual control includes a control lever that is moved forward in accordance with a desired speed forward, that is moved rearward in accordance with a desired speed in reverse, that is moved both forward and to one side to make a turn while moving forward, and that is moved directly to one side to make a pivot turn.
One problem with prior art designs is that control of speed and direction has been uncertain because of the lack of dynamic braking. For instance, when the control lever has been positioned to make a left turn by reducing the electrical power to the left motor, inertia of the wheelchair and occupant has driven the left motor through the drive train that connects the left motor to the left wheel, and the wheelchair has not turned at the desired radius.
A second problem is that it has been necessary to engage and disengage the mechanical drive that connects the motors to their respective wheels, in order to manually propel electric wheelchairs. This has increased design complexity and manufacturing costs.
Commonly a driving connection between the motors and the wheels has been accomplished by using drive rollers that engage the tires. Engagement and disengagement of this driving connection has been accomplished by movement of the motors and mechanical drives, and by resultant movement of the drive rollers into, and away from, engagement with the tires, or by belt tighteners.
A third problem is that disengagement of the mechanical drive has left the wheelchair in a dangerous run-away condition in situations where someone has inadvertently forgotten to set the parking brake. That is, prior art designs have provided neither an automatic parking brake nor an automatic dynamic brake that would restrain the wheelchair from dangerous runaway conditions.
A fourth problem is that when a person with severe hand tremors has tried to control the positioning of the control lever, his hand tremors have moved the control lever rapidly from one side to the other, giving signals for first one and then the other motor to rotate faster, resulting in rapid, and even dangerous, turns in one direction and then the other.
A fifth problem has been a relatively poor overall efficiency of the drive trains that connect the electric motors to respective ones of the wheels, so that an unnecessarily large and heavy battery has been required.
A sixth problem is that prior art designs have been heavy and unwieldy to transport. This has drastically reduced the mobility of handicapped persons, limiting employment possibilities or limiting their opportunities to visit away from their homes or care facilities.
However, if prior art designs of electrically propelled wheelchairs had used drive trains with better efficiencies, then the ability of the wheels to drive the motors through the more efficient drive trains would have provided less dynamic braking, and controllability on turns would have been even poorer.
A seventh problem has been poor contact life in the relays that are used to reverse the potentials of the electric motors. This has resulted in frequent repairs and frequent periods of the wheelchair being out of service.
There are thousands of incapacitated people who would be able to gain a greater degree of self reliance, and some would be able to become a part of the work force of their country, if they were able to control some type of self-propelled conveyance.
Thus, the present invention can help handicapped people to gain a better sense of dignity and self-worth, and can help many of them become productive members of society.
3. Disclosure of Invention
The present invention provides a wheelchair, or conveyance, in which a left propulsion motor is continuously connected to a left propulsion element, or wheel, by a first power transmission; and a right propulsion motor is continuously connected to a right propulsion element, or wheel, by a second power transmission.
Electrical power to the motors is separately and variably controlled in response to a manually-positioned control, similar to the type used with computer games.
The control lever is oriented with relation to the conveyance so that moving the control lever forward results in maximum power in the forward direction being delivered to both the left and right motors.
In like manner, maximum power in the rearward direction is delivered to both motors when the control lever is moved directly rearward, power is delivered to the left and right motors in opposite directions and pivotal turns are achieved when the control lever is moved directly to one side or the other, and various percentages of power in forward and reverse directions are provided when the control lever is positioned in various directions, and at various distances from the neutral position.
Manual positioning of the control lever separately and variably actuates the wiper arms of left-propulsion and right-propulsion potentiometers. Each of the potentiometers provides two variable resistances, one from the arm to one leg thereof, and another from the arm to the other leg thereof.
The following description will describe operation for only one of the motor drives, since both sides function the same, and both clarity and brevity are best achieved in this manner.
The right-propulsion potentiometer cooperates with a signal supply voltage of eight volts that is applied across its legs and functions as a voltage divider to provide a right-propulsion signal.
The right-propulsion signal is supplied as the input to two operational amplifiers. When the right-propulsion signal is more than four volts, one of the operational amplifiers provides a forward-rotation signal for controlling the right propulsion motor; and when the right-propulsion signal is less than four volts, the other of the operational amplifiers provides a reverse-rotation signal for controlling the same propulsion motor.
A forward-propulsion comparator receives the forward-rotation signal and cooperates with a first power transistor to actuate a forward-polarity relay. In like manner, a reverse-propulsion comparator receives the reverse-propulsion signal and cooperates with a second power transistor to actuate a reverse-polarity relay. The forward-polarity and reverse-polarity relays control the polarity of the driving voltage that is supplied to the right-propulsion motor.
But, the actual supplying of electrical power, and the varying of the electrical power that is supplied, is controlled by separate means which functions as follows.
The system uses two diodes to receive the forward-rotation signal and the reverse-rotation signal, and to develop a power-control signal. The power-control signal varies from zero to four volts when an attenuation control is adjusted to allow maximum speed and power; and the power-control signal is attenuated to lower maximum voltages when lower maximum acceleration, speed, and power are desired.
A sawtooth generator and the power control signal cooperate with a comparator to develop a pulse-width-modulated control signal whose pulse-width-modulated are proportional to the magnitude of the power-control signal.
The same sawtooth generator also cooperates with a comparator in the left-propulsion circuitry to develop a pulse-width-modulated control circuit that cooperates with other components for driving the left-propulsion motor.
The pulse-width-modulated control signal cooperates with a transistor to provide a pulse and brake signal. The pulse and brake signal is pulse-width-modulated as is the pulse-width-control signal, but is amplified in power.
The pulse and brake signal controls two field-effect transistors. The first field-effect transistor receives the pulse and brake signal and pulses a connection to ground, so that the supply voltage is pulsed to the right-propulsion motor, thereby supplying a pulse-width-modulated driving voltage to the right propulsion motor. The width of the pulses determines the effective driving voltage.
It should be remembered that the polarity of the supply voltage that is applied to the right propulsion motor has been determined by the forward-rotation and reverse-rotation relays, and the first field-effect transistor determines the width of the pulses of the supply voltage that are applied to the right propulsion motor.
The second field-effect transistor cooperates with the pulse and brake signal to short the motor winding of the right propulsion motor during at least a portion of the intervals that separate the voltage pulses of the pulse-width-modulated driving voltage.
This shorting of the motor windings during a portion of the intervals between pulses of driving voltage causes the right propulsion motor to operate as an electrically loaded generator, and to provide dynamic braking.
However, if the motor winding were shorted for even a small portion of the time when pulses of the driving voltage were being applied to the motor winding, severe damage would be done to the circuit components. Thus, a delay circuit is provided that prevents this occurrence.
The delay circuit includes diodes, resistors, and the parasitic capacitance of the field-effect transistors, and provides a time-interval between the end of one pulse of the pulse-width-modulated driving voltage and shorting of the motor winding.
The delay circuit also provides a time-interval between the cessation of shorting the motor winding and the start of the next pulse of the effective driving voltage.
The present invention includes a differential-limiting circuit for limiting the rate of change in the difference of power that is delivered to the left-propulsion and right-propulsion motors, while leaving the change in the rates of power that can be delivered substantially unaffected when the rates of change of power to both motors are generally equal.
In the preferred configuration, a capacitor, which is connected across the arms of the two potentiometers, limits the rate of change in the control voltages that are provided by the two potentiometers. However, when the control lever is positioned to equally increase or decrease the power to both motors, the voltages of the right and left propulsion signals change equally and the capacitor does not see a difference in differential voltage. Thus, the differential limiting circuit does not affect acceleration or deceleration when changes in electrical power are substantially equal to both propulsion motors.
Limiting the rate of the difference in power delivered to the two motors provides a conveyance that can be controlled by people having severe hand tremors; because spurious signals produced by hand tremoring are time-averaged.
In addition to dynamic braking and differential change limiting, the present invention provides extended relay life, and provides dynamic braking when no pulses of power are being supplied to the motor.
The field-effect transistors cooperate with the relays to pulse the power after the relays are closed, and to cease delivering power before the relays open, thereby avoiding arcing across the relay contacts, and thereby resulting in greatly extended service life for the relays.
In their unenergized state, the relays short the motor winding, thereby achieving power-off dynamic braking even when the battery is removed from the conveyance.
In summary, the present invention provides a conveyance, a motor drive, and a control, in which: the power transmissions continuously connect the motors to the wheels, thereby obviating the necessity of mechanisms to connect and disconnect the power trains; dynamic braking is provided by shorting the motor winding during a portion of the intervals between power pulses, thereby providing superior control for turns and down-grade operation; differential control limiting provides ease and accuracy of control, even for those with severe hand tremors, by limiting the rate of change in the difference of power that can be supplied to one motor with respect to the other motor; and power-off dynamic braking is achieved by shorting the motor winding when no power pulses are being supplied to the motors.
In one embodiment, the present invention provides extended relay life for reversible electric motors by preventing relay contacts from making or breaking contact under load.
In another embodiment, the present invention provides a solid-state switching device in which two electrical connections are alternately made and broken in response to the change in potential in a single conductor, and a delay is provided between the breaking of the one connection and the establishing of the other connection.
Differential control limiting is applicable to both electric and fluid motors; dynamic braking is applicable to any electric motor that is driven by voltage pulses whether width-modulated, amplitude-modulated, or unmodulated; power-off dynamic braking is applicable to various uses, particularly with permanent magnet motors; the circuitry for increasing relay life is particularly applicable to reversible electric motors that are driven by pulsed driving voltages; and the solid-state switching device is applicable to various uses, including reversible electric motors.
According to a first aspect of the invention, there is provided a motor drive having an electric motor, and having a motor control that supplies pulses of a driving voltage to motor. A motor-loading device supplies a plurality of electric motor-loading pulses to the motor, thereby providing dynamic braking for the motor drive.
According to a second aspect of the invention, there is provided a method for providing an electric motor drive with dynamic braking. The method includes: supplying pulses of electrical power to the motor that include intervals therebetween; and placing an electrical load on the motor during a portion of a plurality of the intervals.
According to a third aspect of the invention, there is provided a conveyance of the type having a drive unit that includes an electric motor, and that includes a power transmission that connects the motor to a propulsion element. The improvement includes a motor control that supplies pulses of a driving voltage to the motor; and a motor-loading device that supplies a plurality of electrical motor-loading pulses to the motor, thereby providing dynamic braking for the propulsion element.
According to a fourth aspect of the invention, there is provided a method for dynamically braking a conveyance, which method comprises: drivingly connecting a motor to a propulsion element; supplying pulses of electrical power to the motor that include intervals therebetween; and electrically loading the motor during a portion of a plurality of the intervals.
According to a fifth aspect of the invention, there is provided a motor drive having first and second motors; a motor control that separately and variably supplies power to the first and second motors; a manual control that selectively and variably controls the motor control and the separate and variable supplying of power; and a change limiter that limits the rate of change in differences in power supplied to the first and second motors, and that permits relatively unrestricted rates of change in power supplied to the motors when rates of changes in power supplied to the first and second motors are generally equal.
According to a sixth aspect of the invention, there is provided an electric motor drive of the type having an electric motor that includes a motor winding with first and second ends. A motor control supplies electrical power to the first and second ends of the motor winding, and removes power from the electric motor; and a motor-loading device interconnects the first and second ends of the motor winding when the power is removed from said ends of the motor winding, thereby causing the electric motor to function as an electrically loaded generator; whereby the electric motor provides dynamic braking during power-off conditions.
According to a seventh aspect of the invention, there is provided a reversible electric motor drive of the type having an electric motor, and having a relay that includes electrical contacts for determining the direction of rotation of the electric motor. The improvement comprises: circuitry that closes the electrical contacts of the relay before supplying power to the motor, and that opens the electrical contacts after removing power from the motor, so that arcing between the electrical contacts is obviated and the service life of the contacts is extended.
According to an eighth aspect of the invention, there is provided a solid-state switching device that alternately makes and breaks first and second electrical contacts in response to a change in potential in a single conductor, and that provides a delay between breaking of one of the electrical contacts and making of the other electrical contact, thereby obviating the possibility of instantaneous shorting the solidstate components during switching operations.